Adjust for Firefox >= 90

Find details in the individual commits. Most importantly, this adjust the menu that shows when opening the hamburger menu with Firefox >=90. Instead of being cut off, it has a scrollbar now. While at it, I've adjusted the dialogs to add and edit bookmarks, so they are mostly usable.

Feedback welcome, if there's no feedback I'll just merge this when I find time, in a couple of days or so.